As technologies that improve representable gradations in a pseudo manner by performing frame modulation on input image data having a deeper gradation depth than a gradation depth that can be driven by an image display device, there have been proposed so-called “frame rate control” (hereafter referred to as FRC) technologies, which control the gradations of pixels in each of frames, which are changed in a time series manner.
Patent Literature 1 discloses an image display method that is a technology which displays an image on an image display device on the basis of output data consisting of a smaller number of bits than the number of bits of input image data and that generates output image data having gradations equivalent to the gradations of input image data in a pseudo manner by adjusting the upper bits of the input image data using cyclic state transitions when extending the bit precision.
Referring now to FIG. 1, there will be described an algorithm used in a case where the image display method disclosed in Patent Literature 1 is used and where, for example, the lower 4 bits of 12-bit input image data are used to extend the precision and the upper 8 bits thereof are adjusted.
State transitions having cyclicity shown in FIG. 1 are defined for sixteen states representable by the lower 4 bits to be reduced. A to H are called state signs and show that when “1”, luminance is added to the eighth bit of the upper 8 bits.
In FIG. 1, for description, states IDs 1 to 16 are given to the possible sixteen states of the lower 4 bits. The state number refers to the number of state signs and represents the cycle in which luminance is added (frame number).
For example, when input image data is represented by twelve bits “101110100100”, it is divided into upper bits “10111010” and lower bits “0100” and then a state corresponding to the lower bits “0100” is identified. In FIG. 1, the lower bits“0100” corresponds to the state ID 5, and in this case, one bit is added to the upper bits “10111010” when the state sign is D and H.
That is, when the first frame of the input image data corresponds to the state sign A, the upper bits “10111010” are outputted as output data without change in the first to third frames (corresponding to the state signs A to C). On the other hand, in the fourth frame, the state sign D is 1. Accordingly, one bit is added to the upper bits “10111010”, and “10111011” are outputted as output data.
Also in the fifth to seventh frames (corresponding to the state signs E to G), as in the first to third frames, the upper bits “10111010” are outputted as output data without change. In the eighth frame (corresponding to the state sign H), as in the fourth frame, one bit is added to the upper bits “10111010”, and “10111011” are outputted as output data.
As seen above, when the lower bits are “0100”, one bit is added to the upper bits in the two frames of the eight frames, allowing for representation of an intermediate gradation. That is, assuming that a gradation obtained by adding an extension bit to the upper bits in all frames is 1, when the state ID is 5, one bit is added to the upper bits at a frequency of twice in eight frames. Thus, 2/8 gradation, that is, 0.25 gradation can be represented in an pseudo manner due to an afterimage in human eyes.
The example in FIG. 1 is an example in which eight state signs are used. Accordingly, there is a gap between an ideal gradation (a gradation that has linearity with respect to the state ID and is increased by 1/16 each time the state ID is increased by 1) and an intermediate gradation (a pseudo gradation that can be actually represented in the example in FIG. 1). For example, when the state ID is 2, representing an ideal gradation requires adding an extension bit to the upper bits at a frequency of once in sixteen frames. However, the number of states is up to eight and therefore an ideal gradation is difficult to represent.
In FIG. 1, different state numbers 8, 7, 6, and 5 are set to the state IDs in order to make the intermediate gradations as linear as possible, that is, in order to bring the intermediate gradations as close as possible to the ideal gradations. That is, by setting the state number appropriately, the intermediate gradations are adjusted so as to be increased in steps as close as possible to 1/16. For example, when the state ID is 3, the ideal gradation is 0.125 (= 2/16). For this reason, by setting the state number to 8 and adding one bit to the upper bits at a frequency of once in eight frames, an intermediate gradation 0.125 (=⅛) is realized. When the state ID is 14, the ideal gradation is 0.8125 (= 13/16). For this reason, by setting the state number to 5 and adding one bit to the upper bits at a frequency of four times in five frames, an intermediate gradation 0.8 (=⅘) is realized.